A micro-electromechanical system (MEMS) is a micro-sized electro-mechanical structure manufactured by using microfabrication processes mostly derived from integrated circuit fabrication processes. The developments in the field of MEMS process engineering enabled batch production of electrostatically tiltable MEMS micromirrors and micromirror arrays that can be used in visual displays, optical attenuators and switches, and other devices. Using MEMS devices in fiberoptic switches attracts a particular interest. Light emitted by optical fibers can be focused on micromirrors to reliably switch optical signals between different optical fibers or waveguides.
A significant problem of using MEMS micromirror devices is related to presence of unwanted reflections of light from a fraction of MEMS substrate not covered by micromirrors, such as inter-mirror gaps and mirror hinge structures. While the inter-mirror gaps must be present for the MEMS mirrors to function independently on each other, mirror hinge structures can be hidden by placing MEMS mirrors over the hinges. These “hidden hinge” MEMS micromirror devices are particularly beneficial for operation in wavelength selective optical switches, in which MEMS micromirror arrays are placed in a wavelength-dispersed optical plane. Hiding mirror hinges from impinging optical beams results in efficient stray light suppression.
Hidden-hinge MEMS devices are known. Pan et al. in U.S. Pat. No. 6,992,810 describe a MEMS device having an electrostatic actuator, wherein the actuator's rotor has two pedestals for attaching a rectangular mirror over the actuator, so that the actuating mechanism is completely covered. Nelson in U.S. Pat. No. 6,583,921 describes a hidden-hinge MEMS device having a suspended tiltable platform for non-contacting edge-coupled operation to prevent mirror sticking at an extreme angle of tilt.
Detrimentally, prior-art hidden-hinge MEMS devices have a relatively weak electrostatic actuation force and/or a relatively narrow tilting range. Since electrostatic actuators are accommodated under the tiltable micromirror itself, they are smaller than the micromirror, which limits the achievable torque. Furthermore, at least for fiberoptic switching applications, the MEMS micromirrors have to remain relatively thick to ensure good optical quality (flatness) of the mirror surface. Thicker MEMS micromirrors have higher mass and moment of inertia, which reduces the switching speed. To support a thicker mirror, the torsional hinges have to be thickened as well. The thickened torsional hinges require more powerful electrostatic actuation. However, the electrostatic actuators have to be accommodated under the mirror, within the perimeter of the latter, and thus are limited in length and width. This imposes a limit to which one can increase the actuation force. One can increase the electrostatic force by increasing driving voltage; however, electronic drivers have a limit to which a driving voltage can be increased.
A need exists to construct a hidden-hinge MEMS device that would combine a high switching speed, a good optical quality of the MEMS mirror, and a high electrostatic torque with a relatively large achievable tilt angle of the mirror, without the need to increase a driving voltage of the MEMS device. Accordingly, it is a goal of the present invention to provide such a MEMS device.